The present invention relates to a process for producing a semiconductor thin film formed on a subtrate having an insulated surface. It also relates to a process for producing a semiconductor device having a circuit comprising a thin film transistor (TFT) using such the semiconductor thin film.
The term xe2x80x9csemiconductor devicexe2x80x9d used herein includes all devices and apparatuses that function utilizing the characteristic nature of a semiconductor. Therefore, the semiconductor device used herein includes not only a simple device such as a TFT, but also a semiconductor circuit and an electro-optical apparatus comprising the TFT, as well as an electronic apparatus comprising them as a part.
A technique of producing a TFT by using a semiconductor thin film (having a thickness of about from several tens to several hundreds nm) formed on a substrate having an insulated surface has recently received attention. A TFT is being urgently developed as a switching device of an image display apparatus (for example, a liquid crystal display apparatus).
For example, in a liquid crystal display apparatus, there has been an attempt to apply a TFT to all semiconductor circuits including a pixel matrix circuit controlling individual pixel regions arranged in the form of a matrix, a driving circuit controlling the pixel matrix circuit, and a logic circuit processing data signals from the outside (an arithmetic circuit, a memory circuit and a clock generator).
Recently, a TFT utilizing a crystalline silicon film (polysilicon film) is mass produced owing to its advantage in that a TFT having an operation speed higher than an amorphous silicon film can be produced.
The inventors have studied about a polysilicon film, and have urgently developed a polysilicon film having characteristics closer to a single crystal. As a production process of such a polysilicon film, the inventors disclose a technique described in Unexamined Published Japanese Patent Application No. 9-312260, which in turn corresponds to a pending U.S. application Ser. No. 08/785489. An entire disclosure of Japanese patent Application 9-312260 and U.S. application Ser. No. 08/785489 is incorporated herein by reference.
The publication discloses a method of improving the crystallinity of a polysilicon film on crystallization of an amorphous silicon film, in which a heat treatment at a temperature exceeding 700xc2x0 C. is conducted after crystallization, utilizing nickel as a catalyst element accelerating the crystallization.
However, when a polysilicon film using nickel as a catalyst for crystallization is heat-treated taking into direct contact with an oxidative atmosphere, there are some cases where abnormal growth of silicon oxide (SiCdx) occurs.
The abnormal growth of the silicon oxide appears on the surface of the polysilicon film in the form of a welt. If such silicon oxide is present, a problem arises in that only the silicon oxide is removed during the production process of the TFT, and the silicon is divided.
Therefore, the polysilicon film produced by the technique described in the publication should be avoided from the heat treatment where the surface of the polysilicon film is in direct contact with an oxidative atmosphere. For example, in the case where a thermal oxidized film is formed on the surface of the polysilicon film, measures such as an insulating film covering the surface is required, which becomes a factor of making the process complicated.
The invention has been conducted in view of the problems described above. An object of the invention is to provide a process for producing a semiconductor thin film having high crystallinity without generation of abnormal growth of silicon oxide. Another object of the invention is to provide a semiconductor device having a circuit comprising a TFT using the semiconductor thin film.
The basic constitution of the invention is characterized in that an amorphous semiconductor thin film (typically an amorphous silicon film) is crystallized by using an element selected from the group 14 (particularly preferably germanium) as a catalyst element, and the resulting polycrystalline semiconductor thin film (typically a polysilicon film) is subjected to a heat treatment at a temperature higher than the heat treating temperature on the crystallization (hereinafter called as a crystallization temperature).
A technique of crystallizing an amorphous silicon film using germanium as a catalyst element has been known. The invention is based on the finding in that a polysilicon film having extremely high crystallinity with defects within crystal grains being reduced can be obtained by subjecting the polysilicon film thus obtained to a heat treatment at a temperature higher than the crystallization temperature.
In the technique described in said Unexamined Published Japanese Patent Application No. 9-31226C, there is possibility of forming the abnormal growth of silicon oxide depending on the conditions, but there is no such possibility in the invention. As a result, the complicated procedure, where the surface of the polysilicon film is covered with silicon oxide film on heat-treating at a temperature higher than the crystallization temperature, is not required.
Attention should be paid since germanium has a lower melting point than silicon. When germanium is present in a silicon film in a concentration of 1xc3x971020 atoms/cm3 or more, there is possibility in that the silicon film begins to melt at a temperature of about 900xc2x0 C. Therefore, the crystallization temperature is preferably determined by taking the concentration of germanium added to the amorphous silicon film into consideration.